The people behind the destructive cyberattacks against three banks and two broadcasters in South Korea were likely employees or hired guns of a single organization, a security vendor analyzing the attack code says.

In addition, Symantec says it found a connection between the recent attack and one launched against U.S. and South Korean government institutions in 2011. The link was in the code used to open a backdoor, or communications channel, that enabled the command-and-control server to download files and transmit instructions.

"We believe that they're similar," said Satnam Narang, manager of Symantec Security Response, of the backdoors used in each of the attacks.

Symantec believes the people behind both attacks were just doing their job because of the backdoor file's directory path leading to where the code was compiled on disk in the creator's computer. The path included "Z:\work," an indication that a professional was doing the compiling in a work folder.

"The person who put the code in a folder called work is probably a professional, rather than someone just doing it for the sake of doing it," Narang said. "Based on this we think someone was either paid or ordered to perform these attacks either as a contractor or an employee. That's the implication."

Symantec's analysis of the attack code has yet to produce anything definitive. Rather, the facts uncovered represent baby steps in an investigation that could stretch over months, or possibly years.

The attack, reported last week, involved malware that destroyed the master boot record on the hard drive, causing the infected system to crash and unable to turn back one.

Shinhan Bank, South Korea's fourth largest, reported its Internet banking servers were temporarily blocked, while two other banks, NongHyup and Jeju, said computers at some branches were paralyzed for a couple of hours. Employees at the country's two leading television stations, Korean Broadcasting Systems and MBC, were left staring at blank screens, although normal broadcasts continued.

In the last four years, there has been three major cyberattacks in South Korea, Symantec said. The attack in 2009 and in 2011 was a distributed denial of service (DDoS) assault.

Like in 2013, the malware used to launch the attack in March 2011, called Koredos by Symantec, was also capable of overwriting certain file types and destroying the MBR. Within a Koredos-infected machine, Symantec found a backdoor the company calls Prioxer, which is based on publicly available code.

Other than the malware and backdoor being on the same system, Symantec has not been able to draw a direct link between the two sets of code. However, Prioxer was used in the 2013 attack, because its packer was similar to the data-wiping malware Symantec calls Jokra. A packer is code a hacker wraps around malware to make it difficult to analyze, Narang said.

The attackers in the recent assaults used stolen IDs and passwords for a patch manager software to distribute the malware to PCs in the victims' facilities. While there's no proof, North Korea or another state is suspected of being behind the attack because of its sophistication.